In a liquid crystal display device, a classification based on an operating mode for liquid crystal molecules includes modes such as PC (phase change), TN (twisted nematic), STN (super twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated bend), IPS (in-plane switching), VA (vertical alignment), FFS (fringe field switching) and FPA (field-induced photo-reactive alignment). A classification based on a driving mode in the device includes PM (passive matrix) and AM (active matrix). The PM is classified into static, multiplex and so forth, and the AM is classified into TFT (thin film transistor), MIM (metal-insulator-metal) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type depending on the production process. A classification based on a light source includes a reflection type utilizing natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both natural light and a backlight.
The liquid crystal display device includes a liquid crystal composition having a nematic phase. This composition has suitable characteristics. An AM device having good characteristics can be obtained by an improvement of the characteristics of this composition. Table 1 below summarizes the relationship between these two characteristics. The characteristics of the composition will be further explained on the basis of a commercially available AM device. The temperature range of a nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is approximately 70° C. or higher and a desirable minimum temperature of the nematic phase is approximately −10° C. or lower. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying moving images on the device. Response time that is one millisecond shorter than that of the other devices is desirable. Thus a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1Characteristics of Compositions and AM DevicesNo.Characteristics of CompositionsCharacteristics of AM Devices1a wide temperature range of aa wide temperature rangenematic phasein which the device can be used2a small viscositya short response time3a suitable optical anisotropya large contrast ratio4a large positive or negativea low threshold voltage anddielectric anisotropylow power consumption, a largecontrast ratio5a large specific resistancea large voltage holding ratio anda large contrast ratio6a high stability to ultravioleta long service lifelight and heat7a large elastic constanta large contrast ratio and a shortresponse time
The optical anisotropy of the composition relates to the contrast ratio of the device. A large optical anisotropy or a small optical anisotropy, namely a suitable optical anisotropy, is necessary depending on the mode of the device. The product (Δn×d) of the optical anisotropy (an) of the composition and the cell gap (d) of the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on the type of operating mode. This value is approximately 0.45 micrometer for a device having a mode such as TN. This value is in the range of approximately 0.20 micrometer to approximately 0.30 micrometer for a device having an IPS mode or an FFS mode. In these cases, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large dielectric anisotropy of the composition contributes to a low threshold voltage, low power consumption and a large contrast ratio of the device. A large dielectric anisotropy is thus desirable. The stability of the composition to ultraviolet light and heat relates to the service life of the device. The device has a long service life when the stability is high. These types of characteristics are desirable for an AM device used for a liquid crystal projector, a liquid crystal television and so forth.
A liquid crystal composition including a polymer is used for a liquid crystal display device with a polymer sustained alignment (PSA) type. First, a composition to which a small amount of a polymerizable compound has been added is poured into a device. Next, the composition is irradiated with ultraviolet light, while a voltage is applied between the substrates of this device. The polymerizable compound is polymerized to give a network structure of a polymer in the composition. In this composition, the polymer makes it possible to adjust the orientation of liquid crystal molecules, and thus the response time of the device is decreased and image burn-in is improved. Such effect of the polymer can be expected for a device having a mode such as TN, ECB, OCB, IPS, VA, FFS or FPA.
When a liquid crystal display device is used for a long time, flicker sometimes occurs in the display screen. The flicker relates to image burn-in. It is presumed that the flicker occurs due to the formation of a potential difference between positive and negative frames when it is driven by AC current. An improvement has been tried in order to decrease the occurrence of the flicker in view of the structure of the device or the components of the composition.
A composition having positive dielectric anisotropy is used for an AM device having a TN mode. A composition having negative dielectric anisotropy is used for an AM device having a VA mode. A composition having positive or negative dielectric anisotropy is used for an AM device having an IPS mode or an FFS mode. A composition having positive or negative dielectric anisotropy is used for an AM device with a polymer sustained alignment (PSA) type. An example of a liquid crystal composition having positive dielectric anisotropy is disclosed in Patent document No. 1 described below.
An adjustment of the orientation of liquid crystal molecules is necessary for uniform display characteristics in these liquid crystal display devices. That is, specifically, to orient the liquid crystal molecules on the substrate uniformly in one direction, and to give a uniform angle of inclination (pretilt angle) from the substrate plane to the liquid crystal molecules, for instance. A liquid crystal alignment film plays such a role. The liquid crystal alignment film is one of important elements with regard to display quality of the liquid crystal display device, and the role of the liquid crystal alignment film is becoming important year after year as the quality of the display device is increased.
The liquid crystal alignment film is formed by use of a liquid crystal aligning agent. A liquid crystal aligning agent used mainly is a solution (varnish) of a polyamic acid or a soluble polyimide dissolved in an organic solvent. After this solution has been applied to a substrate, the coating film is transformed to a polyimide-type liquid crystal alignment film by means such as heating. At present, a rubbing method is industrially used to give a function for orientation of the liquid crystal molecules to this film (alignment treatment). The rubbing method is a treatment in which the surface of the liquid crystal alignment film is rubbed in one direction using a cloth planted with fibers such as nylon, rayon and polyester. This method makes it possible to orient liquid crystal molecules uniformly.
In contrast, a photoalignment method has been proposed in which alignment treatment is carried out by irradiation of a photo-reactive film with light, and this method includes photodecomposition, photoisomerization, photodimerization and photobridging (for example, see Non-Patent document No. 1 and Patent documents Nos. 2 to 6). The photoalignment method has advantages in comparison with the rubbing method, where it has a high orientation uniformity, the film is not injured because of the non-contact alignment method, and the cause that generates a poor display of a liquid crystal display device, such as dusts or static electricity, can be decreased, for instance.
Starting materials used for a photoreactive liquid crystal alignment film (hereinafter, sometimes abbreviated to “a photoalignment film”) have been greatly studied. It has been reported that a polyimide, where a tetracarboxylic acid dianhydride, especially a cyclobutanetetracarboxylic acid dianhydride is used as a starting material, orients liquid crystal molecules uniformly and stably (for example, see Patent document No. 2). In this method, a film formed on a substrate is irradiated with ultraviolet light, causing a chemical change to the polyimide and thus giving a function for orientation of the liquid crystal molecules in one direction. However, a photoalignment film prepared by such a method has a possibility that the voltage holding ration is decreased because of an increase in impurity ions for instance, and thus the electrical characteristics is less in comparison with an alignment film subjected to rubbing. A molecular structure of the polyimide has been variously studied to solve this issue (for example, see Patent document Nos. 2 and 3).
It has been pointed that the photoalignment method has a possibility that in a liquid crystal display device, the response time is decreased and the image burn-in is caused, since the anchoring energy of the photoalignment method is small in comparison with that of the rubbing method, and thus the orientation of liquid crystal molecules is poor. We have found a method as described, for example, in Patent document No. 5 that after a polyamic acid has been applied to a substrate and irradiated with light, it is calcined, giving a photoalignment film having a large anchoring energy. However, there is a possibility that the light-transmittance is low and the brightness of a liquid crystal display device is decreased, in a photoalignment film using a polyamic acid produced from a diamine having an azo group as a starting material.